The present invention relates generally to a process and an apparatus for laminating layers of partially thermoplastic films by action of heat and pressure on the layers.
Laminating thermoplastic film under pressure with the action of temperature on other films or other layers, for example paper, creates a firm bond. Along with other uses, this process is used for the preparation of identity cards which have a core layer which can have security prints, personal data, further information and details, and, if appropriate, an image of the cardholder, with the core layer laminated between transparent films for the protection against unauthorized alterations.
As far as identification cards, such as check cards, identity cards or passport cards, are concerned, a multiplicity of security features, such as guilloche prints or watermarks make an imitation more difficult and thus contribute to its avoidance. Security features such as microfilm images, holograms and relief grid images can be used for this purpose as well. These security features are generally inserted in a window hole in the card core and protected by the outer transparent films.
From West German Offenlegungsschrift No. 2,308,876 a plasticized identity card is known, which comprises a thicker, transparent film, a printed special paper of high quality and as far as the securing technology is concerned, which shows differences in thickness resulting from watermarks and/or additionally incorporated security features, and of a thinner transparent film. The thicker transparent film acts as supporting film and confers to the identity card the necessary stability and rigidity as well as the desired thickness. The supporting film can alternatively represent the front or the rear of the plasticized identity card. The thinner transparent film acts as a covering film and may alternatively compose the rear or the front of the identity card.
A photograph in the form of a film transparency can additionally be plasticized in this identity card. In this case the exposed, developed and fixed film transparency is alternatively inserted between the supporting film and the special paper or between the special paper and the covering film and is plasticized together with the films. In addition a signature field of a special paper can be incorporated in the identification card and be plasticized with it. In another embodiment of the identity card, no additional special paper strip is introduced as signature field but an opening in either the supporting film or covering film is left in the covering film during the plastification process and acts as signature field.
During the plastification or lamination process, high pressures and temperatures are required, which endanger the inserted images or holograms. A slight melting of the gelatin layer may occur which could lead, in a photographic information carrier, to image distortions which can end in illegibility of the stored data and information. Holograms are even more sensitive to layer displacement. Layers with relief images are endangered even to a greater extent than are photographic layers during this lamination process. These relief images are produced by exposing and developing photolacquer layers, by embossing thermoplastic films or in an electrophotographic process, by electrostatically charging, exposing and developing a photoconductive, thermoplastic recording layer until a relief image is formed.
To avoid melting of the security features certain precautions have been taken, including eg. embossing the films at temperatures which are as high as possible and are higher than the laminating temperature, crosslinking substances such as aromatic azides being added, thermally or photochemically, to the photoconductive, thermoplastic recording layers. However, these measures are in general insufficient for the image stabilization during the lamination process.